This invention relates to a two-stage differential anodization of valve-metal pellets, and specifically to the use of a nonionic surfactant in the second, higher voltage, stage to eliminate underformed spots.
The two-stage differential anodization process has been described by Bernard and Szpak in U.S. Pat. No. 4,131,520, issued Dec. 26, 1978. In their process, valve-metal pellets are first anodized in a conventional electrolyte during which a uniform film of anodic oxide is formed throughout the pellet structure. In the second stage, a different electrolyte is used to form an outer layer or shell, and the anodization (or formation) is carried out at a higher voltage.
When producing pellet anodes on a large scale using the above method, it was noticed that there were underformed spots in the outer or shell layer. When the valve-metal is tantalum, such spots are easy to detect visually, because of the difference in colors of the anodic oxide formed at different voltages.